The present invention relates to a highly bright coating for phosphor having improved heat resistivity and moisture resistivity, a field luminescent panel with a luminescent layer using coating phosphor for obtaining high brightness and long life time, and a method for applying a coating onto a phosphor for improvement in moisture proof and keeping a high brightness for a long time.
The conventional field luminescent panel has the following structure. A reflective insulation layer is laminated on a back plate. A luminescent layer is further laminated on the reflective insulation layer. A transparent conductive sheet is laminated on the luminescent layer. Leads are provided on the transparent conductive sheet and the back plate. The back plate comprises a metal foil such as an aluminum foil. For the reflective insulation layer, high dielectric powders such as barium titanate are dispersed into an organic binder such as cyanoethylpullulan. For the luminescent layer, phosphor particles are dispersed into an organic binder such as cyanoethylpullulan. The phosphor particles are prepared by activating zinc sulfide with copper, silver and halogen. The transparent conductive sheet may be made of ITO. The above electro-luminance device is further sandwiched between outer coating films such as nitflone having a moisture proof property and further provided with a sheet of nylon having a hygroscopic property between the pane and the outer coating film.
The provisions of the nylon sheet and the outer coating films makes the total thickness of the device increased, whilst it is required to reduce the thickness of the panel.
It is possible to use fleon as a moisture resistive material but is not preferable to use them in view of an environmental pollution control measure. There has not been found out any high moisture resistive material in place of fleon. If the fleon is replaced by other materials but having a lower moisture resistivity than the fleon, it is necessary to increase the thickness of the moisture proof sheets whereby the total thickness of the panel is increased contrary to the requirement for reduction in thickness of the panel. Further, the increase in the thickness of the panel makes the brightness lower.
In order to have the luminescent panel free from any problems as described above, it was proposed to do a direct moisture resistive treatment to the phosphor. Namely, the phosphor is coated with a coating film having a moisture proof. For example, in the U.S. Pat. No. 4,585,673, it is disclosed to form a moisture proof coating film such as aluminum oxide on outer surfaces of the phosphor by a thermal chemical vapor deposition method using a reaction chamber illustrated in FIG. 1.
A cylindrically shaped reaction chamber 1 is placed so that a center axis is directed vertically. The cylindrically shaped reaction chamber 1 has a cylindrically shaped body 1a and a funnel-shaped portion 1b connected with the bottom of the cylindrically shaped reaction chamber 1. A filter 2 is provided at a boundary between the cylindrically shaped body 1a and at the funnel-shaped portion 1b. A first pipe 3 is provided at the bottom of the funnel-shaped portion 1b for supplying a first source gas. The first source gas is supplied to the chamber after heated. A second pipe 4 is provided at an upper position of the cylindrically shaped body 1a of the reaction chamber 1 for supplying a second source gas into the chamber. The second pipe 4 has an expanded nozzle having many discharge holes provided in a peripheral area thereof so that the second gases are injected widely and uniformly. A heater 5 is provided around the cyrindrically shaped body 1a for heating up the phosphor and the first and second source gases. The heater 5 is designed to allow setting various temperatures over time.
The method for applying a coating film onto the phosphor by use of the above reaction chamber will be described as follows.
The reaction chamber 1 is heated by the heater 5 up to a temperature in the range of 60.degree. C. to 150.degree. C. before the phosphor particles 6 are supplied into the reaction chamber 1.
The first gas is supplied through the first pipe 3 into the reaction chamber 1 wherein pressure is adjusted so that the first gas are fluidized over the filter 2. The second gas is supplied through the second pipe 4 and mixed with the first gas. The phosphor 6 is also fluidized over the filter 2 and in the mixed first and second gases. The phosphor 6 is made into contact with the mixed gases and further heated by the heater 5 up to a predetermined temperature but lower than 450.degree. C. which is a critical temperature that the first and second gases are decomposed and reacted. The phosphor 6 is then heated up to a temperature over the above crystal temperature of 450.degree. C., for example, 550.degree. C. or 650.degree. C. to cause the decomposition and reaction thereof so that the phosphor particles 6 in fluidizing state are coated with uniform moisture proof coating films. The supply of the first and second gases and the heating are then discontinued to obtain the moisture proof coating phosphor.
In the Japanese laid-open patent publications Nos. 63-278990 and 1-129090, there is disclosed other method for forming coating films on the phosphor particles, This method will be described with reference to FIG. 2.
A cylindrically shaped reaction chamber 7 is placed so that a center axis is directed vertically. The cylindrically shaped reaction chamber 7 has a cylindrically shaped body 7a and a funnel-shaped portion 7b connected with the bottom of the cylindrically shaped reaction chamber 7. A filter not illustrated is provided at a boundary between the cylindrically shaped body 7a and at the funnel-shaped portion 7b. A pipe 8 is provided at the bottom of the funnel-shaped portion 7b for supplying a first source gas. A high frequency coil 9 is placed in the reaction chamber 7 and connected to a high frequency power source not illustrated. The reaction chamber is contained in a sealed chamber 10 which makes a reduced pressure. A pipe 10 is provided to the sealed chamber and connected to a vacuum pump. The phosphor 11 is supplied in the reaction chamber 7. An infrared lamp not illustrated is further provided to heat the phosphor particles 11 in the reaction chamber 7 up to about 200.degree. C.
The method for applying a coating film onto the phosphor by use of the above reaction chamber will be described as follows.
The sealed container 10 is opened to introduce the phosphor particles 11 into the reaction chamber 7. Subsequently, the sealed container 10 is closed to make the sealed container reduced in pressure at 1.3 to 2670 Pa, preferably 6.7 to 667 Pa.
Via the pipe 8, source gases, for example, mixed gases of nitrogen gas and silane gas are supplied to the reaction chamber 7 to make the phosphor particles 11 fluidizing so as to have entire surfaces of the phosphor particles 11 contact with the source gases.
A high frequency current is applied to the high frequency coil 9 to make nitrogen-containing gas plasma in the coils 9 so as to set the phosphor particles 11 at a predetermined temperature so that moisture proof coating films made of silicon nitride are formed on the phosphor particles 11.
The phosphor particles 11 are then maintained in the above states for a predetermined time in the range of 1-500 min. before the high frequency current application is then discontinued. The supply of the gas via the pipe 8 is discontinued. The sealed chamber is opened to pick up the treated phosphor particles.
If the phosphor particles are coated by the oxide such as aluminum oxide in the manner as described with reference to FIG. 1, it is necessary to rise the temperature of the phosphor particles up to more than 450.degree. C. Zinc sulfide is activated with copper to prepare phosphor which will subsequently be coated with the coating film in the manner as described with reference to FIG. 1. In this case, sulfur may be eliminated from the particles due to the high temperature. There may appear migration or elimination of copper as an activator whereby changing composition of the phosphor and damaging the same. As a result, the brightness is considerably reduced.
If, however, the phosphor particles are coated with silicon nitride in the manner as described with reference to FIG. 2, a high temperature heat treatment over 400.degree. C. is necessary for forming fine coating films over the phosphor particles whereby the phosphor particles receive damages and the brightness thereof is considerably reduced.
In general, the phosphor particles have rough surfaces, for which reason it is difficult to remove pin holes by coating the nitride film such as silicon nitride film.
In the manner described with reference to FIG. 1, the phosphor particles 6 are fluidized by the pressure of the first gas. But this pressure of the first gas provides influence to the proper concentration of the second gas. Namely, the gas pressure for having the phosphor particles has to be adjusted in consideration of not only the amount of the phosphor particles supplied but also the weight of the phosphor particles, wherein the weight is increased due to adhesion of the reaction product. Namely, a complicated adjustment for the amount of the source gases supplied is needed.
On the other hand, if undecomposed source gases are heated at a high temperature over 400.degree. C., the brightness and the coloring quality are deteriorated even the moisture resistivity is not deteriorated.
In the manner as described with reference to FIG. 2, the decomposition and reaction of the source gases appear only in the coil 9, whilst at a position distanced from the coil 9 the phosphor particles are not coated with the reaction product at a sufficient thickness and are even subjected to a high temperature whereby the luminance property is deteriorated. The above local reaction near the coil 9 needs a long time. If the phosphor particles pass through the coil 9 but are biased, then it is difficult to keep constant a distance between the individual particles and the coil 9. This makes it difficult to obtain uniform thickness of the moisture proof coating film even the time of treatment is sufficiently long.
If the diameter of the coil 9 is enlarged to settle the problems, a high frequency power source of a large capacity is needed thereby increasing the cost of the treatment.
As described above, even it is possible to form moisture proof coating films on the phosphor particles, the phosphor particles are heated thereby the brightness is reduced and the life-time is shortened.
Further, the phosphor particles are fluidized by the balance of the gas upstream and the gravity of the particles. If a large amount of the phosphor particles is treated, a high pressure of the source gas is needed to have the large amount of the phosphor particles. A sudden gas pressure application may cause a dispersion of the phosphor particles. In order to prevent this dispersion of the particles, a large reaction chamber is needed and the accessory thereof is also needed to be enlarged.